Coated ammonium nitrate propellants



United States Patent 3,005 692 COATED AMMONIUM NITRATE PROPELLANTS Barnet R. Adelrnan, Los Angeles, Calif., assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed Sept. '21, 1956, Ser. No. 611,367 9 Claims. (Cl. 52-.5)

This invention relates to solid propellant composition containing a hygroscopic compound. In one of its aspects this invention relates to waterproofed or water-resistant propellant composition.

Solid propellant utilized in rockets, such as the type used in assisting the take-off of aircraft, comprise a fuel and an oxidant for oxidizing the fuel. Ammonium nitrate is often utilized as the oxidant while the fuel component is usually a hydrocarbon material which acts as a binder for bonding the oxygen particles into a solid grain as well as acting as a fuel. Binder-fuel materials include asphalt, rubber and other tacky organic, e.g., hydrocarboneontaining, materials.

One of the prerequisite properties of an ammonium nitrate-containing propellant is that it remains relatively stable during transportation and storage. Should any decomposition or deterioration of the propellant occur, the sensitivity of the propellant and the degree of uniformity in performance correspondingly decrease. Deterioration of the propellant can be due to a number of reasons. The elfect of moisture toward a solid propellant is an exceedingly important factor especially when the propellant contains a hygroscopic component such as ammonium nitrate. The afiinity for water of ammonium nitrate is one disadvantage of solid propellants containing same.

Accordingly, therefore, one or more of the following objects will be achieved by the practice of my invention.

One object of this invention is to provide an improved propellant containing a water soluble or hygroscopic component. Another object of this invention is to provide a propellant, the water resistance of which has been substantially improved. A further object is to provide an improved propellant containing a hygroscopic component wherein said propellant is coated with a water resistant composition. Another object is to provide an improved ammonium nitrate containing propellant which is coated with a water-proofing medium fortified with microatomized, non-hygroscopic oxidizing material such as ammonium perchlorate, potassium perchlorate and the like. Other objects will be apparent to those skilled in the art from a consideration of this specification.

The broad aspect of this invention pertains to moistureproofing a solid propellant containing a hygroscopic component by coating said propellant with a moistureproofing material. In one embodiment this invention is directed to rendering an ammonium nitrate containing propellant resistant to moisture by coating the propellant with a water-resisting medium containing micro-atomized ammonium perchlorate or other suitable non-hygroscopic oxidants dispersed therein.

The propellants which are particularly suitable in my invention are those containing a rubbery binder as the continuous phase and a hygroscopic oxidant, particularly ammonium nitrate, as the dispersed phase. The rubbery binder can be natural or synthetic rubber with Mooney values (ML4) generally ranging from about 5 to 100 or higher, preferably from about -to about 40. The natural rubbers and synthetic rubbers are too well known in the rubbers art to warrant detailed discussion thereof.

By way of illustration, however, the synthetic rubbers can be prepared in any manner known in the art such as mass polymerization emulsion. One suitable method is the emulsion polymerization of conjugated diolefins alone or with other polymerization monomers from 0 to 140 F. in such systems as the iron-pyrophosphate, with sugarfree or containing sugar, sulfoxylate and the persulfate recipes. Any suitable emulsifier such as a fatty or rosin acid wax or the like can be used. These recipes usually contain 1 to 9 parts by weight of emulsifier per 100 parts of monomer.

The conjugated diolefins which can be employed are those containing at least 4 carbon atoms per molecule such as 1,3- butadiene, isoprene, piperylene, methylpentadiene, 2,3-dimethyl-1,3-butadiene, chloroprene, 2,3-dimethyl-1,3-hexadiene, 2,3-diethyl-l,3-octadierre and the like. The various alkoxy and cyano derivatives such as 2-methoxy-3-ethylbutadiene, 2-ethoxy-3-ethyl-l,3-hexadiene, 2-cyano-l,3-butadiene and the like are applicable. The monomer which can be copolymerized with the above diene can be any monomer containing an active CH C group such as aryl olefins, esters of acrylic and substituted acrylic acids, nitriles, amides, ketones and Vinylpyridines. Examples of such monomers include, among other, styrene, alpha-methylstyrene, alkyl-substituted styrenes, p-chlorostyrene, p-methoxystyrene, acrylonitrile, methacrylonitrile, methyl methacrylate, butylrnethacrylate, methacrylarnide, methyl isopropenyl ketone, 2-vinylpyridine, Z-methyl-S-vinylpyridine, 3-ethyl-5-vinylpyridine and others. In the preparation of the copolymers, the amount of conjugated diene used is generally in the range from 50 to 99 parts by weight per 100 parts of monomer.

The propellant can have 50 to parts by weight of oxidizer and 50 to 10 parts of binder per parts by weight of oxidizer-binder blend. There can be present the conventional reinforcing agent, plasticizer, antioxidanr, etc. Some reinforcing agents which can be used are, by way of illustration, carbon black, wood flour, lignin, and resins such as the styrene-acrylic acid-divinylbenzene polymers. The antioxidant can be phenyl-beta-naphthylamine, tris-nonyl-phenyl-phosphite, and the like. Any vulcanization accelerator can be employed such as the dithiocarbamatm, e.g., N,N-dimethyl-S-tertiary-butyl sulfenyl dithiocarbamate.

The ingredients are mixed on a roll mill or internal mixer such as a Banbury or a Baker-Perkins dispersion blade mixer. The binder forms the continuous phase in the propellant. The propellant grains are formed by compression molding, injection molding, or extrusion. The grains are then cured at temperatures in the range of 70 to 250 F., preferably between to 180 F. The time for cure is generally around three hours with the higher temperatures and up to 7 days for the lower temperatures.

The particular procedure or method for applying the moisture-profmg material to the propellant is governed by a number of considerations such as the size, type and formulation of the propellant, thickness of the coating desired, the kind of water resisting medium employed, economic factors, etc. The propellant can be coated, i.e., moisture-proofed, by various techniques well recognized in the art such as the one-step and/or multi-step dipping process, brushing or painting or spraying followed by an appropriate drying period of the coated propellant.

The coating medium employed in my invention will preferably have a melting point above about 170 F. and below about 225 F. though compounds having lower or higher melting points can be employed. However, the Air Force specification requirements, temperature at which the coated propellant will be stored, probability of igniting the propellant during the coating procedure and the like, are factors which will limit the melting point range of the particular coating material used.

Among the compounds which can be employed are hydrocarbonaceous waxes, such as paraffin wax, carnauba wax, mont-an WAX, micro-crystalline wax, beeswax, and the like which have melting points in the range from about 170 to about 230 F.; normally solid thermoplastic resins which are fluid at a temperature below about 230 F., preferably those resins which are fluid at a temperature in the range from about 170 F. to about 225 R, such as various polyethylenes, polypropylenes, polystyrene, and the like, and mixtures of the above illustrated waxes and resins; said coating medium containing about 25 to about 50 percent by weight of a micro-atomized non-hygroscopic oxidizing material, based on the weight of said coating medium. Suflicient microatomized oxidizing material such as ammonium perchlorate, potassium perchlorate and the like is dispersed in the coating medium so that the ignition of a propellant so coated will not be adversely afiected, and, in fact, the coating material will burn with a similar uniformity as the propellant composition itself.

The hydrocarbonaceous wax can be fortified with a small amount of a normally solid thermoplastic resin which is fluid at a temperature below about 230 F. Thus, micro-crystalline wax containing up to weight percent, and higher, of a low melting polyethylene can be used as the continuous phase and the micro-atomized, non-hygroscopic oxidant is dispersed therein which generally would be about 25 to about 50 weight percent of oxidant based on the weight of the fortified wax phase.

The average particle size of the micro-atomized, nonhygroscopic oxidant is preferably in the range from about 2 to about 50 microns, though slightly smaller or larger particle sizes can he employed. The non hygroscopic oxidant can be micro-atomized by means of a conventional pulverizing mill, a number of different types of which are commercially available.

The hydrocarbonaceous waxes or mixtures thereof or the normally solid thermoplastic resins which are exemplified above can be dissolved in a suitable organic solvent, such as benzene, toluene, cyclohexane and the like, and sufficient micro-atomized oxidizing material is subsequently added .to the resulting solution. Of course, the organic solvent employed must not deleteriously effect the propellant which is to be coated, i.e., moistureproofed. For example, a normally-solid polyethylene which is fluid at a temperature below about 230 F. is dissolved in cycle-hexane and micro-atomized ammonium perchlorate is subsequently dispersed in the resulting solution, the temperature of said solution being kept below 230 F. The solution will contain up to 10 weight percent, or higher, of polyethylene, based on the weight of the cyclohexane solvent, and between about 25 to about 50 weight percent of dispersed ammonium perchlorate, based on the weight of the polyethylene. Of course, the previously exemplified normally solid thermoplastic resins can be heated to the fluid state below a temperature of 230 F. and the micro-atomized oxidant is then dispersed therein. The propellant is immersed in this fluid mixture, removed, and placed on a rack or other suitable means so that a thin film of solid polyethyene interspersed with micro-atomized oxidant coats the surfaces of the propellant thereby water-proofing said propellant.

Example A high melting paratfin wax (melting point, 18519l F.) is heated to an oily-like fluid to a temperature in the range from about 2l02l5 F. Approximately 32 percent by weight of ammonium perchlorate, based on the weight of said parafiin wax is dispersed in the fluid wax, and the resulting mixture is stirred to maintain the ammonium perchlorate in suspension. A rocket propellant comprising butadiene/lO methylvinylpyridine as the binder and ammonium nitrate as the oxidant is completely immersed in the fluid mixture. The propellant is removed in one minute, placed on a rack, and allowed to dry at room temperature for two hours. A thin, Waterproofed coating of paraifin wax interspersed with microatomized ammonium perchlorate is formed on the propellant. The coating is approximately 5 thousandths of an inch thick.

In general, the moisture-proofed coating can be as much as & inch or greater; however, a thickness in the range from about 2 mils to about 50 mils is especially suitable. Because of the explosive nature of the propellant composition and the micro-atomized oxidant, the temperature of the parafiin wax or other wax or plastic which are employed during the coating step will be generally kept below about 230 F., and preferably below about 215 F.

Reasonable variations and modifications can be carried out in the light of the above disclosure without departing from the spirit and scope of said disclosure.

I claim:

1. An ammonium nitrate-containing rocket propellant grain coated with a mixture consisting essentially of micro-atomized, non-hygroscopic oxidizing material selected from the group consisting of ammonium perchlorate and potassium perchlorate dispersed in a compound selected from the group consisting of hydrocarbonaceous waxes, normally solid polystyrene resin, and normally solid thermoplastic polyloweralkene resins, said resin being fluid at a temperature below about 230 F., and mixtures thereof, said oxidizing material being present in a sufficient amount to provide a coating having burning properties similar to said propellant.

2. An ammonium nitrate-containing rocket propellant grain rendered resistant to moisture by a coating consisting essentially of about 25 to about 50 weight percent of a micro-atomized, non-hygroscopic oxidizing material selected from the group consisting of ammonium perchlorate and potassium perchlorate dispersed in a substance selected from the group consisting of hydrocarbonaceous waxes, normally solid polystyrene resin, and normally solid thermoplastic polyloweralkene resins, said resins being fluid at a temperature below about 230 F., and mixtures thereof.

3. The composition of claim 2 wherein the oxidizing material is ammonium perchlorate dispersed in microcrystalline wax.

4. The composition of claim 2 wherein the oxidizing material is ammonium perchlorate dispersed in a high melting paraflin wax.

5. The composition of claim 2 wherein the oxidizing material is ammonium perchlorate dispersed in polystyrene.

6. The composition of claim 2 wherein the oxidizing material is ammonium perchlorate dispersed in polyethylene.

7. The composition of claim 2 wherein the oxidizing material is ammonium perchlorate dispersed in microcrystalline wax fortified with up to 10 weight percent polyethylene, based on the weight of said micro-crystalline wax.

8. A process of water proofing a solid ammonium nitrate-containing rocket propellant grain, which process comprises coating said propellant with a compound selected from the group consisting of hydrocarbonaccous Waxes, normally solid polystyrene resin, and normally solid thermoplastic polyloweralkene resins, said resin being fluid at a temperature below about 230 F and mixtures thereof, said compound containing about 25 to 50 weight per- References Cited in the file of this patent UNITED STATES PATENTS Lawson Apr. 25, 1939 Carr Aug. 16, 1949 FOREIGN PATENTS Great Britain Oct. 11, 1950 Great Britain Nov. 14, 1951 

1. AN AMMONIUM NITRATE-CNTAINING ROCKET PROPELLANT GRAIN COATED WITH A MIXTURE CONSISTING ESSENTIALLY OF MICRO-ATOMIZED, NON-HYGROSCOPIC OXIDIZING MATERIAL SELECTED FROM THE GROUP CONSISTING OF AMMONIUM PERCHLORATE AND POTASSIUM PERCHLORATE DISPERSED IN A COMPOUND SELECTED FROM THE GROUP CONSISTING OF HYDROCARBONACEOUS WAXES, NORMALLY SOLID POLYSTYRENE RESIN, AND NORMALLY SOLID THERMOPLASTIC POLYLOWERALKENE RESINS, SAID RESIN BEING FLUID AT A TEMPERATURE BELOW ABOUT 230*F., AND MIXTURES THEREOF, SAID OXIDIZING MATERIAL BEING PRESENT IN A SUFFICIENT AMOUNT TO PROVIDE A COATING HAVING BURNING PROPERTIES SIMILAR TO SAID PROPELLANT. 